I'm gonna explain my problem in several steps:
What I want to do: measure a voltage difference between two pins of a sensor (max diffÃ©rence 5V)
Problem: I can't fix no pin from the sensor to the GND or VCC, the sensor being already powered by another device which GND is not the same as mine. Moreover, I don't want to disturb the output of this sensor (as you can guess...)
My question: what would be the problem if I use the differential mode of the ATMega644 to know the voltage difference between the two sensor pins? What happens if one of the two pins is higher than my VCC / lower than my GND?
More info: this sensor is actually a variable resistor being powered by another device (voltage divider). This other device (a car ECU) doesn't have the same ground as me.
Thanks a lot for you help :)